A common electrical contact used in modem devices is the metal-semiconductor contact. Depending on the materials, the contact may be ohmic or rectifying. An ohmic contact has low electrical resistance, regardless of the direction of current flow. A rectifying contact behaves as a diode in that it conducts current freely in one direction but has a barrier to current flow in another. This so-called Schottky barrier is the potential necessary for an electron to pass from the metal to the semiconductor, and it is an important parameter in determining the electrical properties of the metal-semiconductor contact.
Recently, advanced semiconductor devices use a metal-semiconductor, Schottky contact to form the source and/or drain of a MOSFET. The Schottky source/drain (S/D) MOSFET has numerous benefits for achieving many device scaling goals for the 45 nm node and beyond. The metal/silicide structure of the S/D has lower resistance, and it is atomically abrupt. This leads to faster device speed and a scalability advantage over conventional impurity doped S/D technologies. The metal silicide S/D forms a Schottky barrier to the channel, which leads to reduced Ioff leakage. Schottky S/D technology reduces the required dopants in the channel region, thereby leading to higher channel mobility. Furthermore, the Schottky S/D process may include current state-of-the-art CMOS technology, including CMOS on SOI, strained Si techniques, metal gate and high-k gate dielectrics, SiGe strain techniques, as well as other semiconductor fabrication technologies.
Despite these clear advantages, there are many challenges facing the integration of Schottky S/D technology with current manufacturing methods. For example, a large Schottky barrier at a source electrode can significantly degrade the drive current of a Schottky CMOS. In order to solve this problem, source junctions with a Schottky barrier lower than about 0.2 eV are required. Several new S/D materials, such as ErSi2 for NMOS and PtSi for PMOS, have been investigated. However, such material integration has not always been successful. New materials require re-optimization of existing fabrication steps such as metal deposition, silicidation, and etching before Schottky S/D methods can reach mass production.
In light of these and other problems, there remains a need for improving Schottky S/D manufacturing methods. The electrical properties of the metal-semiconductor need to be controlled, but new methods should not cause excessive integration problems with other fabrication methods. One way forward is to develop new Schottky S/D methods that use existing materials, thereby minimizing the integration issues that have hampered conventional methods.